shield tabjump a is much faster, since you are able to do the nair much earlier and even buffer it
if you press up you need to get your controller back in the neutral position, which takes some time and it´s extremly hard to do it frameperfect (way to hard, just to get a nair OoS)
Ive got a special type of using the c-stick by controlling it with the lower half of my thumb (I press the jump button and instantly pull my thumb downwards a centimeter do use the c-stick with the lower half of my thumb)